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Navigation Processing Verification
Chapter 1: Introduction
Chapter 2: Raw Echo Sounder vs P1 Water Depth
Chapter 3: Raw Bird Depth vs P1 Receiver Depths
Chapter 4: Raw DGPS vs P1 Vessel Position
Chapter 5: Raw rGPS vs P1 Buoy Positions
Chapter 6: Raw Acoustic Range vs P1 Receiver Positions
Chapter 1: Introduction
What to look out for
In reality, raw data contains issues such as drop outs, spikes and reflections of the surface of the water. As long as there is enough redundancy of good data in the system, then this bad data will be adjusted to fit in with the good data. The size of the adjustment made to the raw data is called the residual.
Limitations
Contractors generally use a variable speed of sound to calculate the range that is used as the input to their best fit model. They will break up the acoustic networks into regions which are large enough to statistically generate a ‘best fit’ speed of sound. This is a speed of sound which best generates ranges that in turn fit together geometrically. The speed of sound is another variable of the system. Unfortunately, these slightly adjusted propagation velocities are not available to us. This means that non-zero residuals of approx. < 1% are not of concern, as long as these are consistent with all the ranges in that area of the network.
Where acoustic pods are off the end of the streamers, then we have to interpolate their positions between the final receiver and the tailbuoy using a straight line. In reality the streamer will not be a straight line, so our reverse engineered range is not a good test of contractors best fit model. However, this is not a big issue as it only affects ranges off the end of the receivers.
Chapter 2: Raw Echo Sounder vs P1 Water Depth
.png) Overview
The water depth in the P1 is calculated by the contractor using the vessel echo sounder, and other calibration values such as propagation speed, echo sounder draft and tide values. In Havian you compare the contractor's P1 water depth to a Havian calculated version based on data from the P2.
Calculation Method
Starting from the raw depth data, you can optionally
•Apply the Calibrated propagation speed found in the P2 header
•Add the echo sounder draft relative to the vessel’s navigational reference point.
When compared to the contractor’s P1 value, you can verify that the desired processes have been applied.
Limitations
Might have used a propagation speed other than the one defined in the P2 header.
Chapter 3: Raw Bird Depth vs P1 Receiver Depths
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This is a simple overlay of the raw depths of birds in the P2 to the nearest P1 receiver depth. There is no interpolation between adjacent receivers. This attribute will highlight birds that are being rejected from calculation of the depth profile.
Chapter 4: Raw DGPS vs P1 Vessel Position
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Overview
We seek to verify the P1 vessel position using only raw data in the P2.
Calculation Method
Starting from the raw DGPS (lat, lon) data
•Time deskew to shot time
•Apply transformation from WGS84 to local datum
•Projection into UTM zone grid coordinates (Northing and Easting)
•Add Antenna offset relative to the vessel’s navigational reference point using the vessel’s gyro.
•Compare to P1 vessel position in the in-line and x-line directions
Chapter 5: Raw rGPS vs P1 Buoy Positions
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Overview
We seek to verify the P1 buoy position using only raw data in the P2.
Calculation Method
•Add the rGPS range and bearing to the P2 vessel lat/long coordinates.
•Projection into UTM zone grid coordinates (Northing and Easting)
•Add rGPS Antenna offset relative to the vessel’s navigational reference point using the vessel’s gyro.
•Compare to P1 buoy position in the in-line and x-line directions
Chapter 6: Raw Acoustic Range vs P1 Receiver Positions
.png) Overview
To compute the P1 positions, the contractor will use the raw acoustic ranges to find a best fit model for the source and streamers. In this attribute, we check that the contractors P1 positions match the raw acoustic range data that they were derived from.
To do this, we calculate what are called ‘reverse engineered’ P1 ranges. These are ranges that have been back-calculated from the contractors P1 positions. They will show what the contractor’s system adjusted a range to, during the best fit process. In an ideal world, all of the raw data would agree with each other, there would be no need for any adjustment, and the residuals (which are defined as the difference between raw and adjusted) would all be zero.
In the chart above, the red line is the raw P2 range, and the blue line is the reverse engineered P1 reverse engineered range that has been calculated by the Havian software.
Calculation Method
The acoustic pods’ data in the P2 is given as a time difference. We multiply this value by the propagation velocity defined in the P2 header to give the P2 range in metres.
To calculate the reverse engineered range, we must first generate P1 positions for all of the acoustic pods from the positions that are available in the P1. For the acoustic pods on the streamer, we can derive their positions by interpolating the receiver positions, which are usually spaced at about 12m. Where acoustic pods are off the ends of the streamer, we will interpolate between the last receiver and the tail buoy positions. For acoustic pods on buoys, we will apply any fixed offsets from the centre of the buoy based on line heading. Once all derived P1 acoustic pod positions are known, then the reverse engineered ranges are simply calculated as straight line distance between each of them.
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